1. Field of the Invention
The present invention generally relates to a technology for correcting a skew. The present invention specifically relates to a technology for performing deskew to correct a skew.
2. Description of the Related Art
With recent progress in high bit-rate data-transmission in an optical communication system, transmission capacity is increasing. To achieving high bit-rate data-transmission, for example, wavelength division multiplexing can be used in which a plurality of signals is transmitted at different wavelengths.
In the wavelength division multiplexing, delay time due to optical fiber transmission is different with respect to each wavelength. Therefore, even if a plurality of signals each having a different wavelength are simultaneously transmitted from a transmission side, the signals do not always reach a reception side at the same time. A difference in transmission delay time of signals generated in the wavelength multiplexing method is generally referred to as skew.
FIG. 5 is a schematic for explaining a skew that occurs in a conventional optical fiber transmission system. The conventional optical fiber transmission system includes a transmitter 10 that transmits optical signals, and a receiver 20 that receives optical signals, which are connected to each other via an optical fiber 30.
The transmitter 10 includes an 8B10B encoder 11, an electro-optic converter 12, and a wavelength multiplexer 13. The 8B10B encoder 11 8B10B-encodes signals received via respective lanes (lanes 1 to 4). The electro-optic converter 12 converts an electric signal into an optical signal. The wavelength multiplexer 13 wavelength-multiplexes the optical signal.
The receiver 20 includes a wavelength demultiplexer 21, an opto-electric converter 22, and an 8B10B decoder 23. The wavelength demultiplexer 21 wavelength-demultiplexes a wavelength-multiplexed optical signal into signals of respective wavelengths. The opto-electric converter 22 converts an optical signal into an electric signal. The 8B10B decoder 23 8B10B-decodes the signals and performs deskew to correct a skew.
It is assumed herein that signals A5, B5, C5, and D5 of different wavelengths are simultaneously transmitted by the transmitter 10. Because of signal transmission delay characteristics of the optical fiber 30 depending on the signal wavelength, phases of the signals received by the receiver 20 are shifted, which generates a skew as shown in FIG. 5.
For example, when the transmission rate of the transmitter 10 is 10 Gb/s (the transmission rate of each lane after 8B10B encoding is 3.125 Gb/s), and an optical signal with a wavelength of 1.3 micrometers is transmitted via a 10-kilometer long single mode fiber (SMF), a skew of 1.4 nanoseconds (for about 4 bits) occurs.
Therefore, a specific synchronization symbol is embedded in a transmission signal by 8B10B encoding on the transmitter 10 side so that deskew is performed for compensating a skew between synchronization symbols detected on the receiver 20 side. As a result, a skew of up to about 30 nanoseconds can be compensated. Reference may be had to, for example, The Institute of Electrical and Electronics Engineers, Inc, “IEEE Std 802.3ae-2002, IEEE Standard for Information exchange between systems-Local and metropolitan area networks-Specific requirements, Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Later Specifications, Amendment: Media Access Control (MAC) Parameters, Physical Layers, and Management Parameters for 10 Gb/s Operation”, pp. 289 to 310, Online, Aug. 30, 2002, retrieved from the Internet on May 25, 2006, <URL: http://standards.ieee.org/getieee802.3.html>.
In the conventional technology, however, when a modulation rate is increased or a transmission distance of the optical fiber 30 is extended, skew is increased to some extent. This makes it difficult to perform deskew appropriately.
For example, when the transmission rate of the transmitter 10 is 40 Gb/s (the transmission rate of each lane after 8B10B encoding is 12.5 Gb/s), and an optical signal with a wavelength of 1.5 micrometers is transmitted via a 40-kilometer long SMF, a skew of 60 nanoseconds occurs, which exceeds the skew compensation capacity of the 8B10B encoding.
FIG. 6 is a schematic for explaining problems in a conventional deskew process. In FIG. 6, it is assumed that synchronization symbols S1 and S2 are simultaneously transmitted in different wavelengths from the respective lanes 1 to 4 by the transmitter 10.
For convenience of explanation, the synchronization symbols S1 and S2 are shown differently. However, the synchronization symbols S1 and S2 are equivalent in practice, and the receiver 20 cannot distinguish the synchronization symbols S1 and S2 transmitted at different time points from each other.
The receiver 20 receives the synchronization symbols S1 and S2, and detects a skew. At this time, the receiver 20 determines the synchronization symbols S1 and S2 in each lane received closely in time to be the synchronization symbols S1 and S2 transmitted simultaneously by the transmitter 10, and detects a skew.
As shown in FIG. 6, when an excessive skew has occurred, the receiver 20 detects a skew from reception time of the synchronization symbols S1 and S2. Hence, a skew cannot be discerned correctly, and a correct deskew cannot be performed.
One approach to solve this problem could be to employ an encoding method more complicated than the 8B10B encoding. However, if a complicated encoding method is employed, the signal transmission rate increases, resulting in an increase in power consumption of the transmitter 10 and the receiver 20, or the time taken to achieve synchronization increases.
Thus, there is a need of a technology for performing deskew effectively without complicated encoding even when a large skew has occurred.